Yesterday I looked at the anatomy of SEG-Y files. But it's pathology we're really interested in. Three times in the last year, I've heard from frustrated people. In each case, the frustration stemmed from the same problem. The epic email trails led directly to these posts. Next time I can just send a URL!
In a nutshell, the specific problem these people experienced was missing or bad trace location data. Because I've run into this so many times before, I never trust location data in a SEG-Y file. You just don't know where it's been, or what has happened to it along the way — what's the datum? What are the units? And so on. So all you really want to get from the SEG-Y are the trace numbers, which you can then match to a trustworthy source for the geometry.
Easy as 1-2-3, er, 4
This is my standard approach to loading data. Your mileage will vary, depending on your software and your data.
- Find the survey geometry information. For 2D data the geometry is usually in a separate navigation ('nav') file. For 3D you are just looking for cornerpoints, and something indicating how the lines and crosslines are numbered (they might not start at 1, and might not be oriented how you expect). This information may be in the processing report or, less reliably, in the EBCDIC text header of the SEG-Y file.
- Now define the survey geometry. You need a location for every trace for a 2D, and the survey's cornerpoints for a 3D. The geometry is a description of where the line goes on the earth, in surface coordinates, and where the starting trace is, how many traces there are, and what the trace spacing is. In other words, the geometry tells you where the traces go. It's variously called 'navigation', 'survey', or some other synonym.
- Finally, load the traces into their homes, one vintage (survey and processing cohort) at a time for 2D. The cross-reference between the geometry and the SEG-Y file is the trace or CDP number for a 2D, and the line and crossline numbers for a 3D.
- Check everything twice. Does the map look right? Is the survey the right shape and size? Is the line spacing right? Do timeslices look OK?
Where to get the geometry data?
So, where to find cornerpoints, line spacings, and so on? Sadly, the header cannot be trusted, even in newly-processed data. If you have it, the processing report is a better bet. It often helps to talk to someone involved in the acquisition and processing too. If you can corroborate with data from the acqusition planning (line spacings, station intervals, and so on), so much the better — but remember that some acquisition parameters may have changed during the job.
Of vital importance is some independent corroboration— a map, ideally —of the geometry and the shape and orientation of the survey. I can't count the number of back-to-front surveys I've seen. I even saw one upside-down (in the z dimension) once, but that's another story.
Next time, I'll break down the loading process a bit more, with some step-by-step for loading the data somewhere you can see it.